NLM Quiz no. 18: What Is This, and Why Is It On the Grounds of a Church?

It has been several months since our last quiz, so as a reminder of our regular procedure: Please give your answer in the combox, along with any and all details you think pertinent to it. To keep it more interesting, please leave your answer before reading the other comments. We are always pleased to hear humorous answers as well. The structure you see in the photo below is certainly old, but not broken, and not part of a ruin.

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